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Time Travel: Machines
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Wells' famous time travel novel was later made into a full-length film, and The Time Machine (1960, US) has become one of the best-known science fiction films. In the beginning of the film, a well-conceived explanation of the fourth dimension is given by George, a late Victorian-age scientist/hobbyist who has invented a time machine. This scientific explanation sets the stage for the technology at hand: the time machine. This machine is Victorian in style, fitting in well with the film's overall form.
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As one of the foremost scientists in the field of time travel, Princeton astrophysicist Gott takes it upon himself to disseminate advice on building time machines. The construction of the vessel itself is rarely of concern here; it is the way it is used and the way that space-time (the dimensions of space and time that we collectively consider to be our universe) behaves around it that may eventually allow adventurers to break with the usual order of things. Believing that science fiction often spurs true scientific discovery, Gott explores numerous theatrical and literary concepts before moving on to current bona fide theories, pointing out the difficulties of each method. Some possibilities for leaving the present involve dismantling Jupiter, making use of "cosmic strings," taking a trip at near light-speeds far out into space then back and warping space-time itself. Einstein's theory of relativity, upon which all of the presented theories depend, is described in impressively clear language. Practical tips for chrononauts on their options for travel and the contingencies to prepare for make everything sound bizarrely plausible.
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This led to a rash of popular publications claiming, to the annoyance of many relativists, that time travel might be possible. In the 1980s, though, Kip Thorne, of CalTech (one of the world's leading experts in the general theory of relativity), and his colleagues set out to prove once and for all that such nonsense wasn't really allowed by Einstein's equations. They studied the situation from all sides, but were forced to the unwelcome conclusion that there really was nothing in the equations to prevent time travel, provided (and it is a big proviso) you have the technology to manipulate black holes. As well as the Kerr solution, there are other kinds of black hole time machine allowed, including setups graphically described as "wormholes", in which a black hole at one place and time is connected to a black hole in another place and time (or the same place at a different time) through a "throat". Thorne has described some of these possibilities in a recent book, Black Holes and Time Warps (Picador), which is packed with information but far from being an easy read. Now, Michio Kaku, a professor of physics in New York, has come up with a more accessible variation on the theme with his book Hyperspace (Oxford UP), which (unlike Thorne's book) at least includes some discussion of the contribution of researchers such as Robert Heinlein to the study of time travel.
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Representative Adam Smith is ... a vehement time travel-denier, yet he has clearly gone back in time to alter the course of American history - yes, he's THAT Adam Smith. Steve Jobs is the owner of an iTime Machine™, but has so far refused Stephen's request for usage. That, and he hasn't yet given him a free iPhone. The cheap bastard. You'd think all those Pixar movies would warm his heart to such generosity.
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A second type of time travel paradox is the closed causal loop. The classic example is the one in which you one day receive the instructions on how to build a time machine from a mysterious stranger. You follow the instructions and then use the time machine to go back in time, where, disguised as a mysterious stranger, you proceed to give your younger self instructions on how to build a time machine. But where did the knowledge of how to build such a machine come from in the first place?
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A proposed time-travel machine using a wormhole would (hypothetically) work something like this: A wormhole is created somehow. One end of the wormhole is accelerated to nearly the speed of light, perhaps with an advanced spaceship, and then brought back to the point of origin. Due to time dilation, the accelerated end of the wormhole has now experienced less subjective passage of time than the stationary end.
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